Low flow-high pressure centrifugal pump

ABSTRACT

Low flow centrifugal pump that develops more than 25 percent greater pressure than equivalent standard type spiral impellers with circular casings.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the earlier filing date ofU.S. Provisional Application No. 61/550,640, filed on Oct. 24, 2011, thecontents of which are incorporated herein by reference as if set forthin their entirety.

BACKGROUND OF THE INVENTION

There has been a market for very low flow centrifugal pumps used on lowviscosity liquids such as water, chemicals or hydrocarbons, but theseproducts have not been available.

The centrifugal pump basically consists of a rotating impeller within avolute as shown in FIG. 1. The majority of the volutes are designed likea logarithmic spiral as shown in FIG. 1 a. The shape of the impeller andvolute depends on the combination of desired flow, pressure and speed.In the industry of centrifugal pumps, the combination of the threecriteria in a specific equation known throughout the industry is calledspecific speed (Ns). The range of this dimensionless number is 500 toover 4000. Pumps with Ns of 500 have a casing that has a narrow widthand spiral shape that for all intents and purpose is a circle. Incontrast, a pump with Ns of 4000, has a casing width that is relativelywide and the spiral is similar to Nautilus shell as shown in FIG. 2. Toobtain this shaped casing, it is usually made from a casting. Within thespiral, as the impeller rotates, 25% of the pumped liquid is collectedin the first 90 degrees of the spiral, 50% is collected by 180 degrees,75% at 270 degrees and 100% at the 360 degrees or end of the spiral.Where the spiral ends, is called the throat. This is where, in theory,100% of the liquid goes into the discharge nozzle. The bottom of thethroat is called the cutwater. This is also the beginning of the spiral,hence the name. The casing of a 4000 Ns pump has the biggest spiral, butthe least circular leakage. At the other end of the range, a pump with500 Ns speed or less has so little spiral that the inside of the casingis made circular as shown in FIG. 3. By making the volute circular, theinside surface can be machined resulting in a smooth surface, whichincreases the efficiency.

The intersection of the circular shape casing with the tangentialdischarge hole has no defined cutwater, just a tangential hole to thecircle. This produces a lot of circular leakage; resulting in lowhydraulic efficiency. However, even with the low efficiency there isstill a market for a low flow pump.

Similarly, the impellers of a specific speed range change shape with achange in number. A 4000 Ns impeller is relatively wide and with aswooping profile from the inlet to the periphery; while one with a 500or less Ns is very narrow with the passage ways almost radial to theshaft centerline (FIG. 4). Traditionally, the impellers of 4000 Ns mayhave 7 to 8 spiral vanes that include 100 to 120 degrees of thecircumferential profile. 500 or less Ns have only 3 to 4 vanes with longspiral vanes that may encompass 270 degrees of the profile. In thedesign of a pump there are hydraulic force that are produced that createhydraulic forces that react on the support bearing of the shaft. Theseare axial forces that are parallel to the shaft, radial forces that actperpendicular to the shaft and coupling forces that acts as a twistingtorque on the end of the shaft.

The axial force can be the greatest force. To reduce it, a designer mayemploy an enclosed impeller, which means that the hydraulic vanes of theimpeller are between two disks, called shrouds. As the liquid exits thevane the hydraulic pressure goes down the sides of the shrouds andcounter acts the axial thrust one either side, reducing the thrust. Thisis a good solution for pumps that have Ns above 1000. Below that, theratio of width to impeller diameter, number of vanes and wrap around ofthe vanes makes it very difficult to make a casting. Sometimes a twopiece impeller can be molded from a polymer and then attached together.The cost of the molds, cleaning techniques, and quality of theattachment cannot always justify the costs. Another option is to designthe impeller as an open impeller. This utilizes only one shroud, whichhas an open thee so the cast or molded vanes are fully exposed. Becausethere is only one shroud, a high portion of the developed pressure ofthe vanes only is on back side of the impeller and results in high axialthrust. Designers reduce the pressure acting on the shroud by scalloping(cutting out material of the shroud). This does results in negativefeatures; mechanically it is difficult to maintain the flatness of theimpeller, it also weakens the support of the vanes, hydraulically itreduces the efficiency of the pump. The open face of the vanes isnormally machined to have a flat surface. This surface is matched to aflat corresponding stationary surface. The clearance between therotating impeller and stationary surface is about 0.10″. When thisclearance is not maintained the hydraulic performance and efficiency isreduced.

This whole process gas more difficult to produce when the Ns is 500 orless.

BRIEF DESCRIPTION OF THE INVENTION

This present invention relates to pumps with a specific speed of lessthan 500. A pump according to the invention has a circular casing withreplaceable cutwater with close clearance to the periphery of theimpeller that increases the developed pressure. The inlet for the casinghas a singular suction, with a double suction impeller. The doublesuction impeller reduces axial thrust. However, a singular suctioncasing is less expensive and easier to install than a casing designedspecifically for a double suction impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe appended drawing figures wherein like numerals denote like elements.

FIG. 1 shows a volute and spiral vane impeller;

FIG. 1 a shows the basic profile of a spiral volute;

FIG. 2 shows the profile and end shape of volutes for various specificspeeds;

FIG. 3 shows the profile of a circular volute;

FIG. 4 shows the change in impeller profile with change in specificspeed;

FIG. 5 are left side and front views of the double suction impeller;

FIG. 6 is a composite view of the replaceable cutwater and its variousfeatures;

FIG. 7 shows the assembly of the circular volute with a replaceablecutwater and single inlet, double suction, radial slot withrecirculation slots impeller;

FIG. 8 a shows an impeller with spiral vanes;

FIG. 8 b shows an impeller using slots;

FIG. 9 is a photograph of a test set-up using an apparatus according tothe present invention

FIG. 10 a and FIG. 10 b are photographs respectively of the back and thefront surface an impeller according to the present invention;

FIG. 11 a and FIG. 11 b are respective side views of the replaceablecutwater according to the present invention; and

FIG. 11 c is a top view of a replaceable cut water according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to pumps in the Ns range of 300 to 400,which is below previous limit of 500, requiring the invention of a newtype of volute, discharge nozzle, and impeller.

A single end suction casing, overhung close coupled centrifugal pumpwith an open impeller was obtained. The 1.25″ suction nozzle was reducedto 0.625″ to accommodate a garden hose. The 1″ discharge was reduced toa 0.25″ tube. The axial position of the tube in the nozzle could beadjusted.

An impeller was made from a disk that had a width to diameter ratio of0.052. It had four radial slots from the inlet diameter to peripherymachined on the front face and then 60 quarter inch deep radial slotsput around the periphery. The feature of radial slots to develop head isnew, but usually there is a thin membrane in the middle of the slot toguide the liquid from both sides up to the middle of the volute. This isexpensive and is accomplished with a gear cutter. This is method ofincreasing pressure is call regeneration. This developed a head a littlegreater than the standard impeller with a flow 2 gpm. There was arelatively large gap between the front and back of the disk and themating surfaces of the casing and casing cover, resulting in internalslippage and leakage of the liquid. These surfaces were built up withdisks of polymer so the front and back clearances were in the range of0.020 to 0.040″. The developed pressure increased. At this point theimpeller was an open front with 60 slotted teeth. This was developingsubstantial axial thrust resulting in the impeller rubbing the casingcausing the motor to trip out. Some balance holes were added in thedisk, but there was a significant drop in pressure. A new impeller ofthe same width and diameter was made. This one had four radial holesdrilled into the body of the disk. These replace the four radial slots.This made it a closed impeller and eliminated the axial thrust. Thesmall diameter- long holes are difficult to drill without breaking outinto the outer surface of the disk. In addition to the holes the 60slots were replace with 20 special shaped slots that looked like theteeth of a circular saw. This is similar to U.S. Pat. No. 3,746,467dated Jul. 17, 1973. The developed head was less than the 60 slots, butthe flow doubled. A circumferential groove half the width of theimpeller was put in the middle of the teeth. The groove was deep as thebottom of the teeth. The teeth now looked somewhat like the teeth in the'467 patent. The pressure did not increase. The groove was refilled withepoxy. The tube in the discharge was pushed forward so it was 0.010″from the impeller periphery. The pressure increased. The end of the tubewas cut on a bias; this too resulted in an increase in head. Thecircular saw teeth were filled back to look like radial teeth. Thepressure increased.

Since the radial holes are difficult to make, open radial slots wereselected but with a change. The front face of two opposite holes weremachined and then the impeller was turned over and machined to open theother two opposing holes. These last two terminated at the circumferenceof the back hub. Where they terminated, axial holes were drilled intothe suction inlet in the front resulting in a double suction impellerwith radial slots that was basically axially balanced. It also put themechanical seal under suction pressure. The resulting pressure was alittle less than with the closed impeller. The flow was greater.

The difference between 20 degree spiral vanes and radial vanes wastested. Four radial slots were filled in with an epoxy resin and fourspiral slots were machined into the impeller. The spiral slots ranquieter but produced less pressure resulting in a return to the radialslots. More radial slots were added which increased the pressure. Thenumber of teeth was doubled by vertically band sawing the existingteeth. This resulted in a large increase in pressure.

The resulting impeller is a suction double suction, double sided openimpeller with radial slots on the periphery as shown in FIG. 5.

The casing also went through a number of changes and additions. The tubein the discharge nozzle was replaced with a replaceable cutwater that isset screwed into position. The replaceable cutwater can be adjusted tomaintain the close clearance between it and the periphery of theimpeller as shown in FIG. 6. It is made of a material that resistserosion and corrosion.

The side profile of the volute has dimensions and shape to guide theliquid from the impeller to split to both sides of the volute and guideit back into the impeller to regenerate the pressure of the liquid asshown in FIG. 7.

The impeller according to the present invention has three by features,namely:

a. Slots vs. vanes: Traditionally centrifugal impellers used wide spaceda spiral shaped passage ways with thin vanes along or within theimpeller disk to develop pressure from the inlet to periphery of theimpeller as shown in FIG. 8. When used in an enclosed impeller it isvery difficult to produce. In an open impeller, the thin vanes are canbe cast, machined or molded. The open space between the vanes results ina lot of internal slippage. This pump uses radial slots instead ofvanes. Therefore, there is less internal slippage (FIG. 8 b). Testing ofthis pump showed that there was little performance difference betweenradial and spiral grooves. The radial groove produced slightly higherpressure. The pump seemed to be quieter with spiral grooves;

b. Double suction vs. single suction: The impeller has a singular inletthat matches the casing inlet. There are radial pumping slots on bothsides of the impeller disk. The front slots are directly connected tothe periphery of the inlet. The back slots terminate at the back hub ofthe impeller. There are axial holes the same width as the slots adjacentto the hub that communicate the back slots to the impeller inlet. Thisallows liquid to be pumped up the back slots. The amount of liquidpumped of the front are about equal, resulting in a minimum of residualaxial thrust.

c Radial grooves vs. spiral vanes: As the impeller rotates it addsenergy to the moving liquid. Some of the liquid is re-circulated withinthe impeller passages. With the discharge valve fully open most of theenergy passes through the pump. As the valve is closed to reduce flow,energy is trapped within the casing and impeller. When the valve isfully or almost closed this energy is changes to heat. The rise oftemperature of the casing and internal parts can result in catastrophicmechanical failure. An impeller with only radial grooves rather thanspiral vanes has much less contained water and absorbed energy,resulting lower internal heat build.

d. Dam vs. no cutwater: The placement of a dam (replaceable cutwater inthis design) in the volute passage where the tangential discharge meetsthe circular volute increases the developed pressure. This principal ofa dam is employed in what is referred to as a regenerative pump. Thesepumps have a circular volute. A regenerative pump has tine teeth alongthe periphery of the impeller. There may be from 40 to over 80 teeth.There is usually a thin radial web between the teeth to act as a guidefor the liquid and a support between the teeth. The present inventiondoes not employ a web. In a regenerative pump the inlet and dischargeconnections are usually adjacent (located at the same diameter) as theteeth. A close fitting permanent dam separates the two connections. Theprincipal is that the liquid comes into the teeth it is dischargedradial into the volute, but is forced back into other passing teeth.This cork screwing of the liquid in and out of the teeth continuouslyincreases the pressure of the liquid as it goes around to the volute andexits out the discharge. These pumps required dose side and damclearances. The dam wears down from erosion caused by the pressure breakdown from discharge to suction ports. Since this pump has a circularvolute with a dam (replaceable cutwater), radial teeth without a radialweb were added to the periphery of the impeller. These teeth are fed bythe liquids coming from the radial grooves are both sides of theimpeller as shown in FIGS. 5 and 7.

The assembly of the impeller in the casing is shown in FIG. 7. It showsthe single inlet in the casing, the liquid going into both sides of theimpeller, the radial teeth on the periphery and the replaceable cutwaterin the casing.

A pump according to the present invention was constructed and tested asshown in FIG. 9. After all adjustments were made, the increase inpressure was from 10 to 30 percent over a standard design centrifugalimpeller.

FIGS. 10 a and 10 b are photographs of the back and front surfaces of animpeller according to the invention.

FIGS. 11 a and 11 b are side views and FIG. 11 e is a top view of areplaceable cut water according to the present invention.

While the principles of the invention have been described above inconnection with preferred embodiments, it is to be dearly understoodthat this description is made only by way of example and not as alimitation of the scope of the invention.

1. A low flow centrifugal-regenerative pump having an impeller disk,said impeller disk having radial grooves on the face of said impellerdisk and radial peripheral teeth on a peripheral edge of said impellerdisk to increase pressure of fluid exiting said pump due to corkscrewcirculation within a volute of said pump and a dam to push said liquidinto a discharge nozzle in said pump.
 2. A low flowcentrifugal-regenerative pump according to claim 1 wherein said dam is areplaceable cutwater to increase developed pressure.
 3. A low flowcentrifugal-regenerative pump according to claim 2 wherein saidreplaceable cutwater Tater has a shape to provide optimal increase inperformance of said pump.
 4. A low flow centrifugal-regenerative pumpaccording to claim 1 having a single entrance impeller with passagescommunicating with an entrance to said radial grooves to effect a doublesuction impeller.
 5. A pump impeller disk having radial grooves toreduce internal flow which results in less internal heat build up at lowfluid flow through a pump fitted with said impeller.